1. Field of the Invention
The present invention relates to an apparatus and method for receiving signals in a wireless communication system; and, more particularly, to an apparatus and method for receiving signals in a wireless communication system using a multi antenna.
2. Description of Related Art
In general, a receiver of a wireless communication system includes a variable gain amplifier (VGA) and an analog-to-digital converter (ADC). The variable gain amplifier is configured to correct the gain of a signal received by the receiver. The analog-to-digital converter is configured to convert an analog signal, outputted from the variable gain amplifier, into a digital signal.
If the variable gain amplifier excessively increases the gain of a signal received by the receiver, the output signal of the variable gain amplifier may deviate from the operation range of the analog-to-digital converter. This may cause a distortion of the received (RX) signal. On the other hand, if the variable gain amplifier excessively decreases the gain of an RX signal, a quantization noise increases, thus degrading the demodulation performance.
In a general multi-antenna system, the gains of RX signals are suitably controlled on an antenna-by-antenna basis. However, in wireless channel environments, the RX signals of the respective antennas differ in their power levels. In some case, the power level of an RX signal in an antenna is 100 or more times higher than the power level of an RX signal in another antenna. In this case, the antenna with a high RX signal power level has a low relative noise level after gain control, while the antenna with a low RX signal power level has a high relative noise level after gain control. That is, the antenna with a high RX signal power level has a high signal-to-noise ratio (SNR), while the antenna with a low RX signal power level has a low SNR.
As described above, the respective antennas of a multi-antennal system have different SNRs according to wireless channel environments. However, when gain control is performed by an automatic gain controller (AGC) and a variable gain amplifier, the output signals of the variable gain amplifier have similar average power levels. When the signals with similar average power levels are inputted into a multiple input multiple output (MIMO) system, the signal demodulation performance degrades, particularly when they are applied to a linear equalizer with low implementation complexity.
Also, when the bandwidth of an RX signal decreases or increases (i.e., varies) after the gain control by the variable gain amplifier, the voltage gain reference value of the variable gain amplifier must also vary accordingly.